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256                                           THE EAR

close to the surface. For this reason a mastoid incision in young children
should not be extended in a downward direction otherwise the facial nerve
may be cut just after emerging from its foramen. The mastoid or tympanic^
antrum which is present from birth communicates anteriorly-via the aditus"
with the epitympanic recess, and behind and below with the mastoid air cells.
In the infant it lies at a higher level than in the adult and is superficial, being
covered only by a thin plate of bone. The roof of the tympanic antrum is the
backward extension of the tegmen tympani. The floor and medial wall are
formed by the petromastoid bone, the medial wall being related to the bony

The mastoid cell system shows a considerable normal variation both in size
and number of cells. Where the cell system is extensive the mastoid is referred
to as pneumatic or cellular while one composed mainly of sclerotic or dip-
loeic bone is termed an acellular or dense mastoid. The cells are lined by

Fig. 135. Horizontal section through right temporal bone, viewed from above. 1, Posterior vertical
canal; 2, Posterior fossa of skull; 3, Vestibule; 4, Vestibular nerve; 5, Cranial opening of perilymphatic
aqueduct; 6, Cochfear nerve; 7, Cochlea; 8, Inferior petrosal sinus; 9, Internal carotid artery; 10,
Dura mater of middle cranial fossa; 11, Tensor tympani; 12, Zygoma; 13, Meniscus of lower jaw-joint;
14, Inner surface of tympanic membrane; 15, External acoustic meatus; 16, Air cells between antrum
and meatus (border cells); 17, Facial nerve; 18, Floor of tympanic antrum; 19, Sigmoid sinus.

very thin flattened epithelium. They form two groupssquamomastoid and
petromastoid. In an extensively pneumatized mastoid the groups form a
definite arrangement and are designated anatomically: (1) Zygomatic cells
extending into the root of the zygomatic process; (2) tegmen cells bordering
the floor of the middle fossa; (3) petrosal angle cells lying in the angle between
the roof and posterolateral wall of the mastoid cavity; (4) marginal cells lying
posterior to the sigmoid sinus plate and sometimes extending into the
occipital bone; (5) perisinus cells overlying the lateral sinus plate; (6) peri-
antral ceHs related to the antrum; (7) retrofacial cells extending round and
underneath the facial nerve; (8) perilabyrinthine cells extending in the bony
wall of the internal auditory meatus as far as the petrous apex; (9) tip cells
(lateral and medial divided by the groove for the posterior belly of the
digastric muscle) occasionally extending into the styloid process; and (10)